To allow optical drives to accurately process blank (i.e. unwritten) recordable optical media, recordable optical media are manufactured with one or more detectable characteristics to provide optical drives with tracking, timing and absolute (as opposed to merely relative) addresses. Recordable optical media such as Compact Disc-Recordable (CD-R), Digital Versatile Disc-Recordable (DVD-R), Blu-Ray Disc-Recordable (BD-R), etc., generally comprise one or more sides having one or more layers with a continuous spiral track formed by a continuous radiating spiral pre-groove (or channel) that is separated by a continuous radiating spiral land (or mesa). Depending on the particular optical media, data may be stored in or on the groove(s) and/or land(s). Other media may comprise concentric circular, as opposed to continuous spiral, tracks.
The pre-groove is a detectable characteristic that supports tracking. Another detectable characteristic common to most recordable optical media is a periodic deviation of the pre-groove from a perfect spiral, or circle, which is referred to as a wobble. The wobble supports timing, but may also support absolute addressing. Depending on the type of media, the wobble may be fixed or modulated. The modulation of the wobble supports absolute timing or addressing. Alternatively, fixed wobbles may be accompanied by other detectable characteristics such as land pre pits (LPPs) in support of absolute timing or addressing.
An optical pick-up unit (OPU) in an optical drive detects and uses the fixed or modulated wobbled spiral track of recordable optical media to generate a wobble signal, perhaps along with other signals related to other detectable characteristics. The wobble signal and/or alternate or additional signals are used to produce Absolute Time In Pre-groove (ATIP) and/or Address in Pre-groove (ADIP) data for optical drives. ATIP and/or ADIP data is used by optical drives to spin optical media at a constant linear velocity (CLV) and precisely determine the position of their OPU along the track on the media either by sector or time.
ATIP and/or ADIP data is also used by optical recorder drives to synchronize data recorded on optical media with the wobble or other characteristic(s). As such, during playback (i.e. reading) of recorded optical media in an optical drive, the wobble or other characteristic is not required as a separate signal for an optical drive to maintain timing or addressing because the recorded data provides the same information as the wobble signal. However, for blank areas of optical media, the wobble or other characteristic remains the only indication of timing and addressing.
Thus, in order to continuously maintain timing and position in recordable media having both written and blank areas, the optical drive may need to switch back and forth between processing the wobble signal and the recorded data. An unexpected transition between written and blank areas on optical media may, as in the case of an optical drive having only one channel, result in a read error in the optical drive, causing a time-consuming read-retry to regain a lock on timing and position. Transitions between written and blank areas may also contribute to inaccurate tracking or radial error (i.e. REN) signals. Transitions may also contribute to inaccurate gain control (e.g., saturation due to excessive amplification of blank areas).
Therefore, it would be advantageous to detect modulation transitions in an optical storage medium to avoid time-consuming read-retry's, inaccurate tracking and/or over/under amplification due to unannounced transitions between written and blank areas on optical media.